In many systems, soft start function is provided for monotonic start-up of the systems, to reduce the impact to the systems during the start-up and thereby avoid overload to some elements thereof and over voltage of the systems. For further detailed information about soft start, readers may refer to U.S. Pat. No. 6,552,517 to Ribellino et al. In conventional soft start methods, the system always starts up from a zero voltage. However, there may be some electric charges remaining on the output terminal of the system after the last operation of the system, which makes the output voltage of the system not be zero at the beginning of the next start-up. Conventionally, the soft start of a system always releases the remaining electric charges on the output terminal of the system in advance and then charges it again, which wastes those remaining electric charges. For not wasting the remaining electric charges, U.S. Pat. No. 6,841,977 to Huang et al. provides a soft start circuit to keep the low side switch off until the output voltage reaches a target value, so as to remain the residual voltage on the output. However, this soft start circuit can be only used in a buck pulse width modulation (PWM) circuit. U.S. Pat. Publication No. 2004/0228152 to Solie provides a soft start circuit to charge a soft start capacitor by the residual voltage on the output of a voltage converter before enabling the voltage regulation, which will have the voltage on the soft start capacitor equal to that on the output due to charge sharing. When enabling the voltage conversion, the voltage on the soft start capacitor is used as a reference voltage to be compared with the voltage on the output terminal, so there is no need to release the residual voltage on the output terminal. However, this soft start circuit needs an extra capacitor as the soft start capacitor, and the sawtooth wave at beginning to be compared with the output voltage has a RC discharge curve, which is disadvantageous to the performance of the soft start.
FIG. 1 shows a synchronous switching regulator 100 with a soft start function, and FIG. 2 is a waveform diagram showing the signals in the circuit of FIG. 1. Referring to FIGS. 1 and 2, waveform 200 represents the output voltage Vo of the synchronous switching regulator 100, waveform 202 represents the ramp signal SS_Ramp for soft start function, waveform 204 represents the feedback signal VFB, waveform 206 represents the enable signal EN for the error amplifier 104, waveform 208 represents the enable signal Real_EN for the PWM controller 106, waveform 210 represents the high side signal Source_CTL, waveform 212 represents the low side signal Sink_CTL, and waveform 214 represents the inductor current IL. In the synchronous switching regulator 100, the high side switch 108 and the low side switch 1 10 are serially connected between the power input Vin and ground GND, to be switched to produce the output voltage Vo and the inductor current IL to supply for two loads 112 and 114, and the sample circuit 116 samples the output voltage Vo to produce the feedback voltage VFB into the inverting input of the error amplifier 104. At time t1, the enable signal EN changes to high as shown by the waveform 206 to enable the error amplifier 104, the synchronous switching regulator 100 enters a soft start mode, and the multiplexer 102 switches the ramp signal SS_Ramp to the non-inverting input of the error amplifier 104. If there is a residual voltage on the output terminal Vo, the feedback signal VFB will not be zero from the beginning, so the ramp signal SS_Ramp at beginning will be lower than the feedback VFB, as shown by the waveforms 202 and 204 in FIG. 2. In this condition, the PWM controller 106 will not be enabled, in order to keep the residual voltage on the output terminal Vo. At time t2, the ramp signal SS_Ramp rises higher than the feedback signal VFB, thereby making the error amplifier 104 trigger the enable signal Real_EN to enable the PWM controller 106, the enabled PWM controller 106 produces the PWM signals Source_CTL and Sink_CTL according to the error signal S1 provided by the error amplifier 104 to switch the switches 108 and 110 respectively, and thus the output voltage Vo will rise up from the residual voltage on the output terminal. After the soft start finishes, the multiplexer 102 switches the non-inverting input of the error amplifier 104 to receive a reference voltage Vr. However, at time t2, the error signal S1 just begins to rise up, so the duty cycle of the high side signal Source_CTL is small, and the duty cycle of the signal Sink_CTL is large, as shown by the waveforms 210 and 212 in FIG. 2, which causes a reverse current IL, as shown by the waveform 214 between time t3 and time t4, and further makes the output voltage Vo to decrease, causing energy loss, as shown by the waveform 200 between time t3 and time t4 in FIG. 2.
Therefore, it is desired a circuit and method for soft start of a switching regulator from a residual voltage and prevention of a reverse current during the soft start period.